Radio frequency generator



Oct. 12, 1954 B. BOYD ETAL RADIO FREQUENCY GENERATOR Filed Dec. 7, 1948 LOW . INVENTORS Bruce Boyd, Sheldon I. Rambo and hazard H Hagopian.

BY g ATTORNEY Patented Oct. 12, 1954 UNITED STATS CI. v

RADZEQ FREQUENCY GENERATOR Bruce Boyd, Sheldon I. Rambo, and Richard H.

Hagopian, Baltimore, Md, assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Application December "Z, 1948, Seriai No. 64,012

Claims.

Our invention relates to high-frequency generators, and more particularly to high-frequency generators for use with induction and dielectric heating apparatus.

In apparatus constructed in accordance with the prior art, an attempt was made to maintain the power density, that is the power per unit volume, in the high-frequency load constant regardless of load magnitudes with a stabilizing system including a saturable reactor or a controlled induction regulator. Either system is costly and the latter is slow in responding. Because of the high cost of the stabilizing highfrequency heating apparatus of the prior art requiring stabilization is frequently not competitive with heating apparatus of other types. The lag in the response is an additional handicap; because of it the material to be heated, for example, a batch of raw rubber or gears under heat treatment, may be scrapped because of burning or underheating. In these days of material shortage such a mishap is a serious matter.

Apparatus typical of the prior art is shown in United States Patent No. 2,175,694. In apparatus shown in this patent, the power output of an RF generator is controlled from a thyratron which responds to a signal derived from the generator. This signal is proportional to the output current of the generator. However, the apparatus disclosed in the above cited patent is not entirely efiective in that it responds only to an increase above a pre-set level and in particular the arrangement shown and described is not applicable where dielectric loads are treated.

It is, accordingly, an object of our invention to provide low cost apparatus for maintaining the power density in the load constant regardless of variations in load magnitude insofar as the power density is affected by a change in the applied voltage; that is, the high-frequency generator controlled as herein described is a con stant voltage source (has zero regulation) Consequently, the network adjustments necessary to obtain the desired constant power density can be more easily made. Obviously, when a power density is desired which is not constant but varies as a function of load magnitude or characteristic of the material, the network adjustments can also be more readily made.

A further object of our invention is to provide apparatus, which shall have a rapid response, for maintaining the power density in the load constant regardless of variations in load magnitude insofar as the power density is afi'ected by a change in the applied voltage.

A still further object of our invention is to provide apparatus that shall maintain the tank voltage of a high-frequency generator constant.

Still another object of our invention is to provide dielectric heating apparatus that shall maintain the RF tank voltage of the high-frequency generator constant with wide variations in quantity and characteristics of the material being heated.

Still another object of our invention is to provide induction heating apparatus that shall prevent the RF tank voltage and, hence, oscillator grid current from increasing beyond a pro-set maximum.

Still a further object of our invention is to provide dielectric heating apparatus that shall simultaneously maintain the power density of the load, the material under treatment, constant regardless of variations in load magnitude, insofar as the power density is afiected by a change in the applied voltage, and the RF tank voltage of the high-frequency generator constant with wide variations in quantity and characteristics of the material being heated.

Another object of our invention is to provide apparatus including an RF generator which shall automatically reduce the voltage adjustment of the generator to a minimum value when the power is removed from the generator.

Another object is to provide a means for operating the cathode of the oscillating tube or tubes at a minimum voltage consistent with the power demand at any given time, so as to secure a much longer life for the oscillating tube or tubes than if the cathode were maintained constant at full voltage.

Another object is to provide a means for main taining the RF tank voltage of an HF generator constant with normal variations in the supply voltage to the generator.

Our invention arises from the realization of a need for a system to protect the gene-rating apparatus from changes in voltage due to load or power supply variations, such systems being responsive to either increase or decrease in voltage. The system of the above cited patent is not effective because it responds only to decreases i the power load.

In accordance with our invention, we provide an RF heating system in which the power density is maintained constant, insofar as the power density is affected by a change in the applied voltages, by controlling the RE generator in accordance with the resultant derived by a1gebraically subtracting a signal from the RF heater from a standard signal. In dielectric heating systems, the signal is proportional to the voltage of the RF generator, and the effect of the stabilization is to maintain this voltage constant.

in the specific practice of our invention, the signal voltage rectified appears at one of the windings of the polarized relay and opposes the standard on the other winding. Any unbalancing as a result of variation in load, for example, softening of the material or adding or removing material, causes a voltage regulator to restore the RF tank voltage to the initial value. Thus, the power density in the load, the material under treatment, will remain constant regardless of variations in load magnitude, insofar as the power density is affected by a change in the applied voltage.

Apparatus is provided for automatically reducing the control voltage of the generator to a minimum value when the main power source to the generator is removed. This apparatus is provided so that the generator may be started up at reduced voltage since the load presented to the generator at the start may be unknown.

The novel features that we consider characteristic of our invention are set forth with par ticularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will be understood from the following description of specific embodiments thereof when read in connection with the accompanying drawing, in which:

Figure l is a schematic diagram of apparatus and circuits embodying our invention as applied to dielectric heating; and

Figure 2 is a schematic diagram of apparatus and circuits embodying our invention as applied to induction heating.

Referring to Figure 1 the apparatus comprises a generator I which, as specifically shown, is an RF generator having two oscillator tubes 3 and 5. These oscillator tubes have anodes 6 and 6, cathodes and l, and control electrodes 8 and 8. The RF generator I has a plate tank circuit Consisting of a capacitor 9 and an inductance coil 19. Capacitor 9 has two electrodes H and I2 through which the materials under treatment are passed. These electrodes may also be a part of a remote circuit coupled, by means known to those skilled in the art, to the oscillating tank circuit 9, H]. The oscillator tubes 3 and -receive their plate voltage from a rectifier l3, one of the connections from the rectifier l3 being tapped onto the inductance coil ID. A capacitor I4 is connected to this same tap on the inductance coil [9, the other side of the capacitor l4 being grounded. The rectifier I3 is fed from an A.-C. source |5 through a circuit breaker i1 and a transformer l9. This circuit breaker H has contacts 2| and 23, a coil 25, and an on-off switch 21.

There is a connection from the junction of the capacitor 9 and the inductance coil H) to an adjustable capacitor 29. There is also a connection from the other side of the adjustable capacitor 29 to a fixed capacitor 3|. The junction of the adjustable capacitor 29 and the fixed capacitor 3| is connected to the anode 33 of a rectifier tube 31. The adjustable capacitor 29 and the fixed capacitor 3| serve as a voltage divider for sampling the RF tank voltage. One side of the fixed capacitor 3| is grounded and there is a connection from this ground side to the rectifier tube cathode 39 through a current-limiting resistor 4|. Resistors 43 and 45 are connected between the anode 33 of the rectifier tube 31 and winding 47 of the polarized relay 49. Capacitor 59 by-passes resistor 45 and the winding 41 of the polarized relay 49 for RF voltage. The filament 5| of the rectifier tube 31 is supplied by a transformer 52 and is by-passed for RF 4 voltage by two capacitors 53 and 55. The transformer 52 receives its power from the A.-C. source l5. One end of the winding 4'! of the polarized relay 49 is connected to the junction of the resistor 43 and the capacitor 59 through a variable resistor 45, and the other end of the winding 41 isconnected to an ammeter 51. The other end of the ammeter 51 is connected to ground. This ammeter 57 gives an indication of relative RF tank voltage or can be calibrated to read RF tank voltage directly. The ammeter 57 is paralleled by a capacitor 69. The RF tank voltage required for the treatment of different materials varies. The variable resistor 45 is provided so that the voltages on the windings 4'! and 59 of the polarized relay 49 can be balanced for several different tank voltages. A variable current-limiting resistor I60, in series with the winding 59, secures a fine adjustment of the voltage across winding 59. Thus, any desired RF tank voltage can be obtainedand at the same time have the voltages on the windings-41 and 59 of equal magnitude'with the polarized relay 49 in the'neutral position shown in the drawing. In this manner, the desired power density in the material under treatment can be obtained. To aid in the actual balancing of the voltages on the windings 41 and 59 of the polarized relay 49, an ammeter 5|, in series with the winding 59, is provided. Thus, the voltages on the windings 47 and 59 can be adjusted until the readings on ammeters 51 and 69 are equal.

The other winding 59 .of the polarized relay 49 is supplied froma regulated rectifier supply 63 which furnishes a constant voltage. This regulated rectifier supply 93 receives its energy from a transformer 65 which is connected to the A.-C. source 15 through fuses'tl and 58. Selenium rectifier units 69, H, 13 and 15 form a bridge rectifier circuit whose output is filtered by a Pi-network consisting of capacitors 11 and I9 and an inductor 8|. The current-limiting resistor I60 is in series with the winding 59 of the polarized relay 49. In parallel with this resistor [69 and the winding 59 of-the polarized relay49 is a voltage regulator tube 85. This tube maintains a constant voltage across itself for wide changes in line voltage. In series with the current-limiting resistor ISO is a voltage regulating resistor 81.

A group of selenium rectifier units 89, 9|, 93 and 95 form abridge rectifier circuit having its input connected to the transformer 95. The output of this rectifier is filtered bya capacitor 93, and a bleeder resistor 91 is in parallel with this capacitor 96. The contact arm 98 of the polarized relay 49 is adapted to complete the circuit from the bleeder resistor 91 and the selenium rectifier units 89, 9|, 93 and 95 to the motor control relay coils 99 and |0|. These two coils 99 and |0l are paralleled by capacitors H13 and I06, respectively. Capacitors I03 and I95 are used for spark suppressionon the contacts of relay 49. A current-limiting resistor N9 is in series with the motor relay coils 99 and NH when the circuit to these coils is completed.

The contacts I95 and I0! for the motor relay coils 99 and lol'control thedirection of rotation of a reversible motor |||which operates a vari able auto-transformer 3, which in turn adjusts the filament voltage of oscillator tubes 3 and 5. Instead of using a variable auto-transformer l3, an inductionv regulator could be used, the motor forming a part of the induction regulator.

A manual-automatic switch H5 is in the circuit with the motor I! l and when in the position shown the RF tank voltage of the RF generator l is kept constant automatically. When the switch H5 is in the opposite position, the filament voltage and, hence, the RF tank voltage of the RF generator 5 can be either raised or lowered by means of the two manual switches H1 and i 19. The motor H I is provided with motor limit switches 12] and I23.

It is desirable to start up the RF generator I at a reduced output voltage since the load presented to the RF generator I at the start may be unknown. This is accomplished automatically by providing auxiliary contacts I25 and i2! on the circuit breaker H. In the specific apparatus shown, the auxiliary contacts E25 and M! are connected in the circuit so as to return the filament voltage on the oscillator tubes 3 and 5 to a minimum when the plate power to the RF generator l is removed. The contacts I25 and I2! are shown in the plate power on position.

The operation of equipment shown in Figure l is as follows:

Assume that the RF generator i has been adjusted to give its maximum rated output into a particular load. The adjustable capacitor 29 is adjusted to just balance the voltage on the winding 41 of the polarized relay 49 against the voltage on the other winding 59 of the polarized relay 48 which is supplied from a constant source of voltage. The variable resistor 45 can be adjusted so that the voltages on windings M and 59 can be balanced for different values of output of RF generator I. The variable currentlimiting resistor 1166 can be adjusted to give the desired voltage on the winding 59, thus giving a further means of balancing these two voltages. When relay 49 is balanced, the motor relay contacts I05 and ill? will be in the position shown, and the motor 5 i i will not run.

Now suppose that the load presented to the RF generator i is reduced such as would be the case if a smaller piece of material passed between the electrodes II and E2 or the dielectric value of the load during treatment changed. The reduction of load causes the RF tank voltage to rise. Hence, a larger RF voltage appears across the rectifier tube 3! causing more direct current to flow through the winding 4'3 of the polarized relay d9. Thus, the polarized relay 49 is unbalanced in the left-hand position completing a circuit for the motor relay coil llli which when energized closes its contact lill'. This completes the power supply circuit to the motor Hi so that it rotates in a direction to reduce the filament voltage on the oscillator tubes 3 and 5. This reduction in filament voltage lowers the RF tank voltage. As soon as the tank voltage is reduced to the value which it had at the original maximum rated output, the polarized relay-4S again balances causing the motor i H to cease reducing filament voltage. It can be readily seen that if the load is increased, resulting in a decrease in the RF tank voltage, the opposite motor relay coil 99 will be energized by operation of the polarized relay 49 to cause the motor regulator HII l3 to increase the filament voltage to thereby increase and restore the RF tank voltage to the original value.

The RF generator 8 is started up at reduced filament volta e on the oscillator tubes 3 and 5. This is accomplished by the auxiliary contacts H25 and [2! which, as soon as plate power is removed take up a position opposite from that shown which represents the position of the auxiliary contacts when plate power is on. When these contacts take up this opposite position from that when plate power is on, the motor ill will be energized so as to reduce the filament voltage on the oscillator tubes 3 and 5 to a minimum.

These auxiliary contacts I25 and i2 are necessary since without them the filament voltage on the oscillator tubes 3 and 5 would be increased to a maximum as soon as the plate power was removed from the RF generator l since the rectifier tube 3! would be receiving no RF voltage and would signal the motor Hi to call for more filament voltage for the oscillator tubes 3 and 5. With these auxiliary contacts I25 and 52?, the RF generator I, having once been set for the desired RF output voltage, may be turned on regardless of load conditions and will automatically regulate its tank voltage to suit the load presented to it.

Referring to Figure 2, this figure replaces the RF generator I in Figure 1, all other parts of this circuit remaining as shown in Figure 1, and illustrates the case where an RF generator is applied to induction heating. This arrangement comprises a generator 2PM and is specifically shown as an RF generator having an oscillator tube 263. The RF generator 2M has a plate tank circuit consisting of a capacitor 285 and an inductance coil Zil'i. In series with coil 26'! is an applicator coil 269 wherein the material to be inductively heated is placed. The oscillator tube 263 receives its plate voltage from a rectifier id as shown in Figure 1. One of the connections from the rectifier i3 is connected to the plate RF choke 2H and is by-passed by capacitor M as in Figure l. The anode of the oscillator tube 283 is coupled to the tank circuit by capacitor 2 Iii. There is a connection from capacitor 295 and the inductance coil 28? to an adjustable capacitor 23 as shown in Figure 1.

The operation of the equipment shown in Figure 2 is essentially the same as described for Figure 1 except that variations in the tank voltage are produced by variations in the amount or characteristics of the material in the applicator coil 2439 instead of variations in the material between the dielectric heating electrodes i l and H2 in Figure 1.

It is to be understood by those skilled in the art that any conventional HF generator or oscillator could be used in the place of the one shown in the drawing.

It is also to be understood by those skilled in the art that some aspects of our invention may be applied to low-frequency generators.

It is to be further understood by those skilled in the art that an induction regulator could be used in the place of the auto-transformer H3 shown. The motor iii and induction regulator could form one unit. Also other equipment could be used for furnishing the standard reference voltage.

In the specific embodiment of our invention, the auto-transformer i It varies the filament or cathode voltage on the oscillator tubes 3 and 5. However, it is to be understood by those skilled in the art that the control voltage could be applied to any of the other elements of the oscillator tubes such as the anode, or control electrode.

Although we have shown and described certain specific embodiments of our invention, we are fully aware that many modifications thereof are possible. Our invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

7 We'claim as our invention: 1. The combination of :a-high frequencygeneratorhaving at least one oscillator tubehaving acontrol voltage applied thereto and adaptedto supply a load voltage, means for maintaining a constant reference voltage, means for opposing said voltage by a voltage proportional to thesaid load voltage of said high frequency generator, and means responsive to the resultant of said opposed voltages for controlling the output of said high frequency generator'by-varying said control-voltage.

2. The-combination-of a high frequency generator adapted to supply a load voltage and com- .prising at least'one oscillator tube having a control voltage applied thereto, a polarized relay having at least two windings, means for applying a constant reference voltage on one of said-windings, means including a rectifier for applying an opposing voltage on the other said winding which is proportional to the said load voltage of :said high frequency generator, and means responsive to the resultant of said opposed voltages for controlling the output of said high frequencygenera-tor by varying said control voltage.

3. The combination of a high frequency .generator comprising at least one oscillator tube having a control voltage applied thereto, a plate power supply for said high frequency generator, a voltage control device whose output controls said control voltage, andmeans for returning the output of said voltage control device to a minimum value when said plate power supply is removed.

4. The combination of a high frequency generator comprising at least one oscillator tube having a control voltage applied thereto, a plate power supply for said high frequency generator, a main breaker:in said plate power supply-line, a voltage control device for controlling said control voltage, and means for-returning the output of said control device to a minimum value when'said plate power supply is removed, said means comprising a reversible motor for operating said voltage con- .trol device, and auxiliary contacts on said breaker, said contactsbeing in circuit with said reversible motor.

5. Thecombination of a high frequency generator adapted to supply a voltage to a load and comprising at least one oscillator tube having a control voltage applied thereto, a plate power supply for said high frequency generator, means for maintaining a constant reference voltage, means for producing a rectified'voltage proportional to the voltage supplied to said load, means responsive to the resultant of said last-mentioned two voltages for controlling the output of said high frequency generator, and means for return- .ing said control-voltage to a minimumvalue when vmentioned two voltages for maintaining the radio frequency tank voltageof said high frequency generator constant by varying said control voltage.

7. Thecombination of a high frequency gen- .erator adapted to supply a load voltage and havingat least one oscillator tube having a control voltage applied thereto, means for rectifying va portion of the output of said high frequency. generator, means for varyingthe magnitude of said rectified output, means for maintaining a constant reference voltage, said last-mentioned means -ineluding means for making said reference voltage adjustable, means for opposing said reference voltage by a voltage proportional to the said load voltage of said high frequency generator, and means responsive to-the'resultant of said opposed voltages for controlling the output of said high frequency generator by varying said control voltage.

8. The combination of a high frequency generator adapted for connection to aload and having at least one oscillator tube comprising at least an anode, a cathode, and a control electrode,

means for maintaining a constant reference voltage, means for producing a rectified-voltageproportional to the output voltage of said high fre quency generator supplied to said load, and

.means responsive to the resultant of said voltages for controlling the output of said high frequency generator by varying the voltage on the cathode of said oscillator tube.

-9. The combination-of a high frequency generator adapted for connection to a load andcomprising-at least one oscillator tube having a control voltage applied'thereto, a plate-power supply for said high frequency generator, means for maintaining a constant reference voltage, said last-mentioned means includingmeans for'making said reference voltage adjustable, means 'for producing a rectified-voltage proportional to the output voltage of said-high frequency generator .which is applied to said load, means for varying the magnitude of said rectifier voltage, and means responsive to the resultant of said-adjusted reference voltage and said adjusted rectified-voltage for controlling the output of said high frequency generator by Varying said control voltage, and means for returning saidlast-mentioned voltage to a minimum valuewhen said plate power supply is removed.

10. In induction heatingapparatus the combination of a high frequency generator comprising at least one oscillator tube having a controlvolt- .age applied thereto, an radio frequency "tank circuit for said generator, an applicator coilconnected'in circuit with said radio frequency tank circuit for passing the material under treatment ,therethrough, means for maintaining a constant reference voltage, means for producing a rectified voltage proportional to 'the output voltage of said high frequency generator which is applied .to

said material, and means responsive to the resultant of said last-mentioned two voltages for maintaining the radio frequency tank voltage of said-high frequency generatorconstant by vary- 'ing said control voltage.

References Cited in the file of thispatent UNITED STATES PATENTS Number Name Date 2,175,694 Jones, Jr. Oct.'10, 1939 2,218,502 Breitenstein Oct. 22,1940 2,415,799 Reifelet al Feb. 11,.1947 2,431,902 Albin Dec. 2, 1947 2,587,175 Lappin Feb. 26, 1952 2,609,510 Wilmotte "Sept. 2, 1952 

